The stated goal of this project is to map the topology of interactions between RecA and UmuD proteins involved in mutagenesis. The precise amino acid sequences that contact each other will be determined by introducing cysteine residues at various locations through site-directed mutagenesis. The cysteines will be derivatived with 14C-labelled p-azidophenylacylbromide, a cysteine-specific photoactivatable crosslinker. Dimers between the proteins of interest will be crosslinked, isolated, and digested into fragments suitable for characterization by mass spectrometry.